ODP: thru zero VCO questions

[Terry Michaels]

Message text written by Martin Czech
>You want to supress a sideband at 1MHz with LC-filters?
Forget it. It will be difficult enough to have the quartz
osc. stable enough, they usually spec to 100ppm (Drift, aging).

HAM people get a (very bad) sideband rejection with mechanical filters
@ 455 kHz (something like -30dB), that's good for radio, but
not for a frequency modulation substitute.

There must be a reason why people use complicated polyphase
or phase difference networks in order to throw
away the unwanted sideband.

And there is certainly a reason why J.H. did it all in base band.

The higher you modulate, the higher are the filter requirements,
and also frequency stability requirements, differences of large values.

m.c.
<

Hi Martin:

You can divide down the quartz oscillators to around 50 KHz, the filtering
will be easier there.  If a master clock is used to divide from, and it
supplies both mixing frequencies, frequency errors will largely cancel out
after translating the VCO up and then down.   Or, an image rejection mixer
using 90 degree hybrids can be used, as you indicated.   There are many
possible ways to do this.

Terry Michaels